Jm. Bekkers et Cf. Stevens, CABLE PROPERTIES OF CULTURED HIPPOCAMPAL-NEURONS DETERMINED FROM SUCROSE-EVOKED MINIATURE EPSCS, Journal of neurophysiology, 75(3), 1996, pp. 1250-1255
1. The passive cable properties of rat hippocampal neurons in dissocia
ted culture were studied using focal application of hypertonic solutio
n to locally elicit miniature excitatory postsynaptic currents (mEPSCs
) on the soma and dendrites. Neurons were filled with Lucifer yellow a
nd portions of their dendritic trees were measured. 2. The average mEP
SC measured at the soma appeared smaller and slower as the site of suc
rose application was made more distal. Normalizing to a 1-mu m diam de
ndrite, the mean mEPSC peak amplitude and charge was reduced e-fold in
170 and 1,000 mu m, respectively, and the mean mEPSC decay time const
ant was increased e-fold in 150 mu m. However, for any particular sucr
ose site, individual mEPSCs varied widely in their amplitudes and time
courses. Plots of individual peak amplitudes versus half-width or ris
e time showed much overlap for mEPSCs originating from sites as much a
s 100 mu m apart. This suggests that use of such plots to estimate the
electrotonic location of synaptic currents is highly prone to error.
3. Averaged mEPSCs recorded when applying sucrose at the soma were poo
rly fitted by an alpha function but were well-described by an equation
of the form m(x)h, where m incorporates a rise-time constant tau(1) a
nd h a decay time constant tau(2). Averaged fits to mean mEPSCs elicit
ed at the somas of five cells gave (mean +/- SE): peak conductance = 8
32 +/- 126 pS, tau(1) = 0.29 +/- 0.06 ms, tau(2) = 3.03 +/- 0.24 ms, x
= 4.7 +/- 0.7. 4. For three cells, the entire dendritic branch to whi
ch sucrose was applied was measured and used to construct a passive ca
ble model. The specific membrane resistance (R(m)) and intracellular r
esistivity (R(i)) were varied systematically in the model (assuming me
mbrane capacitance C-m = 1 mu F/cm(2)) to search for the best agreemen
t between the mean mEPSCs and the model. Optimal R(m) was found to lie
in the range 20-30 k Omega cm(2), R(1) in the range 100-200 Omega cm.
5. These results confirm those obtained by other methods and emphasiz
e the considerable cable filtering of fast electrical events in cultur
ed hippocampal neurons.